1. Field of the Invention
The present invention relates to a piezoelectric lamination actuator used as driving a source, such as for a printer head, a positioning stage in a piezoelectric longitudinal effect.
2. Description of the Related Art
Two types of piezoelectric lamination actuators have conventionally been available: a first type is a lamination actuator made up of a number of piezoelectric layers with both ends of the actuator terminated by dielectric layers; a second type is a multiple-unit actuator which comprises a plurality of the first type of actuators connected together.
As shown in FIG. 14 and FIG. 15, the first type of piezoelectric lamination actuator according to the prior art comprises a laminated piezoelectric body made up of a number of piezoelectric layers 2, each sandwiched between internal electrodes 1. Glued on both ends of the laminated body are dielectric layers 3, one for each end. Two external electrodes 4 are coated on two diagonally opposite sides of the laminated piezoelectric body of the piezoelectric layers 2, with one external electrode connected to every second internal electrode and the other external electrode connected to the rest of the internal electrodes. Lead wires 5 are soldered to the external electrodes 4. The sides of the laminated piezoelectric body of the piezoelectric layers 2 and the dielectric layers 3 are coated with dielectric resin, such as epoxy resin, silicone rubber, in order to keep moisture off, and assure insulated construction.
As shown in FIG. 16, the internal electrodes, which sandwich each piezoelectric layer 2, alternate spatially between a positive electrode 1a and a negative electrode 1b, so that the direction of the electric field reverses spatially in an alternate manner as well. Each internal electrode has, on its one open end, a non-electrode portion.
These piezoelectric layers 2 are stacked in lamination, by gluing them on top of each other with dielectric resin adhesive as shown in FIG. 16. Even though dielectric resin adhesive is employed, the use of press bonding permits conductivity between two internal electrodes. A conductive adhesive agent has conventionally been employed to form the two external electrodes 4, 4 on the two sides of the laminated piezoelectric body of the piezoelectric layers 2. The conductive adhesive agent coated on the laminated piezoelectric body of the piezoelectric layers 2 is subjected to a hot setting treatment, causing it to be connected to internal electrodes as shown in FIG. 16. The external electrodes 4, formed by hot setting the coated conductive adhesive agent, are subject to poor contact with the internal electrodes 1; defective actuators thus frequently result in the prior art actuator.
The external electrodes 4 are mechanically weak, and soldering Joints of the lead wires 5 are weak, as well.
Furthermore, since the dielectric layers 3 on both ends of the laminated piezoelectric body are normally glued or press bonded onto driven bodies, the dielectric layers 3 cannot be covered with a coating material 6. For the purpose of manufacturing efficiency, the same sort of ceramic material as the piezoelectric, layers 2 is commonly employed as the dielectric layers 3. In general, such sort of dielectric ceramic material has a poor moisture resistance characteristic.
The poor moisture resistant dielectric layers 3 allow moisture to intrude and reach internal electrodes 1 or piezoelectric layers 2, thereby deteriorating dielectric strength.
Although thickening the ceramic material employed as the dielectric layers 3 improves moisture resistance, reduced output displacement results while the actuator is in service. It should be noted that the dielectric layers 3 are non-driving members, which do not contribute to the displacement output.
The second type of actuator is now discussed. When an actuator having numerous laminations is constructed using numerous piezoelectric layers 2, for example, the thickness of the actuator without both dielectric layers fitted thereon is about 120 mm if 800 layers are laminated, assuming that each layer is 150 .mu.m and that the actuator is made up of a single body rather than multiple-unit bodies combined.
With these layers glued in a single body construction, the entire actuator becomes unusable if any defects, such as dielectric deterioration, take place in any one single layer out of 800 layers.
The more the number of laminations of piezoelectric layers employed, the smaller the yield rate of actuator production results. A lowered production efficiency also results.
To overcome this disadvantage, the following method is available: several actuator units 7, each having the proper number of laminations of piezoelectric layers which still provide an acceptable yield rate, for example, four actuator units, each having 200 layers with an overall thickness of about 30 mm, may be longitudinally glued together to form a multiple-unit lamination actuator.
Such a construction involves electrically connecting external electrodes of two neighboring actuator units. As shown in FIG. 17, lead wires 8 are soldered on external electrodes 4 to connect the actuator units.
When each lead wire 8 is soldered to electrically connect the neighboring electrodes 4, a projected portion extends outward from the actuator body at each junction where a lead wire 8 is soldered. If the actuator is placed in a metallic housing 9 to protect the lead wires 8 against possible breakage due to buckling as in FIG. 18, the metallic housing should have sufficient internal capacity to accommodate the projected portions. A large metallic housing 9 is thus needed.
Soldering each lead wire 8 involves an increased number of soldering Joints, thereby lowering the production efficiency. Furthermore, each junction point between two neighboring actuator units 7 is mechanically weak.
Each ceramic dielectric layer 3 on both ends of each actuator unit 7 serves as a machining allowance portion to be adjusted so that both ends of the actuator unit 7 are accurately in parallel when the actuator unit 7 is machined, and serves as a protective member against moisture or mechanical damage when the actuator is put into service. The dielectric layers 3 are non-driving members which do not contribute to the displacement output.
Each actuator unit 7 according to the prior art employs dielectric layers 3 which are made of ceramic and machined to equal thickness. If two actuator units 7 are connected, the thickness of internal dielectric layers becomes double. As already mentioned, the dielectric layers 3 do not contribute to the displacement output. The thicker is the total thickness of the dielectric layers, the lower is the output displacement.